U.S. patent number 8,808,919 [Application Number 12/628,586] was granted by the patent office on 2014-08-19 for negative electrode active material, negative electrode having the same and lithium secondary battery.
This patent grant is currently assigned to Samsung SDI Co., Ltd.. The grantee listed for this patent is Bong-Chull Kim. Invention is credited to Bong-Chull Kim.
United States Patent |
8,808,919 |
Kim |
August 19, 2014 |
Negative electrode active material, negative electrode having the
same and lithium secondary battery
Abstract
A lithium secondary battery including a positive electrode
having a positive electrode active material, a negative electrode
having a negative electrode active material, a separator separating
the positive electrode from the negative electrode and an
electrolyte. The negative electrode active material includes a
graphite core particle, a carbon layer coating the graphite core
particle, and metal particles dispersed in the carbon layer.
Inventors: |
Kim; Bong-Chull (Suwon-si,
KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kim; Bong-Chull |
Suwon-si |
N/A |
KR |
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Assignee: |
Samsung SDI Co., Ltd.
(Suwon-si, Gyeonggi-do, KR)
|
Family
ID: |
41647275 |
Appl.
No.: |
12/628,586 |
Filed: |
December 1, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100136432 A1 |
Jun 3, 2010 |
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Foreign Application Priority Data
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Dec 1, 2008 [KR] |
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10-2008-0120514 |
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Current U.S.
Class: |
429/231.8 |
Current CPC
Class: |
H01M
4/133 (20130101); H01M 4/386 (20130101); H01M
4/366 (20130101); H01M 4/134 (20130101); H01M
4/587 (20130101); H01M 4/625 (20130101); Y02E
60/10 (20130101); H01M 10/0525 (20130101); H01M
4/1395 (20130101); H01M 4/1393 (20130101) |
Current International
Class: |
H01M
4/58 (20100101) |
Field of
Search: |
;429/231.8 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1894811 |
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Jan 2007 |
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CN |
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101005130 |
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Jul 2007 |
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CN |
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1 903 628 |
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Mar 2008 |
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EP |
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2000-203818 |
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Jul 2000 |
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JP |
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3124272 |
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Oct 2000 |
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JP |
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3305995 |
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May 2002 |
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2004-59386 |
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Feb 2004 |
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2004-063411 |
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Feb 2004 |
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2005-108774 |
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Apr 2005 |
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2005-310760 |
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2007-184263 |
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JP |
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2007-519182 |
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JP |
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2007-194201 |
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JP |
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2008-186732 |
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Aug 2008 |
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JP |
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2009-181767 |
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Aug 2009 |
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JP |
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10-2004-0082803 |
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Sep 2004 |
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KR |
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10-0453896 |
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Oct 2004 |
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KR |
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10-0589309 |
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Jun 2006 |
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KR |
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10-2007-0012385 |
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Jan 2007 |
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KR |
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10-0684729 |
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Feb 2007 |
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KR |
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10-0738054 |
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Jul 2007 |
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KR |
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10-0745733 |
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Jul 2007 |
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KR |
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WO2005/096333 |
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Oct 2005 |
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WO |
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Other References
European Search Report dated Mar. 3, 2010, in corresponding
European Patent Application No. 09177629.4. cited by applicant
.
Abstract of Japanese Patent No. 2001-160392. cited by applicant
.
Abstract of Japanese Patent No. 10-236809. cited by applicant .
Abstract of Korean Patent No. 10-2001-0113448. cited by applicant
.
Abstract of Korean Patent No. 10-2002-0070764. cited by applicant
.
Abstract of Korean Patent No. 10-2006-0048753. cited by applicant
.
Abstract of Korean Patent No. 10-2006-0069738. cited by applicant
.
Abstract of Korean Patent No. 10-2007-0034254. cited by applicant
.
Fourth Office Action issued on Oct. 12, 2013 by the State
Intellectual Property Office of P.R. China in corresponding Chinese
Patent Application No. 200910246086.5. cited by applicant .
Notification of the Second Office Action issued by the State
Intellectual Property Office of P.R. China dated Oct. 23, 2012, 9
pages. cited by applicant .
Third Office Action issued by the State Intellectual Property
Office of P.R. China dated Apr. 17, 2013 in the corresponding
Chinese Patent Application No. 200910246086.5. cited by
applicant.
|
Primary Examiner: Siddiquee; Muhammad
Attorney, Agent or Firm: Knobbe, Martens, Olson & Bear,
LLP
Claims
What is claimed is:
1. A negative electrode active material, comprising: a graphite
core particle; a carbon layer coating the graphite core particle;
and metal particles dispersed in the carbon layer; wherein the
graphite core particle, the carbon layer, and the metal particles
form a metal-graphite composite having a porosity of from 0.03 to
0.08 cc/g; wherein the graphite core particle has a porosity of
from 0 to 0.025 cc/g; and wherein the carbon layer is coated to an
average thickness of from 1 to 4 .mu.m.
2. The negative electrode active material according to claim 1,
wherein the graphite core particle is formed of one selected from
the group consisting of artificial graphite, natural graphite,
graphitized carbon fiber, graphitized mesocarbon microbeads,
amorphous carbon, and a combination thereof.
3. The negative electrode active material according to claim 1,
wherein the metal particles are formed of one selected from the
group consisting of Cr, Sn, Si, Al, Mn, Ni, Zn, Co, In, Cd, Bi, Pb,
V, and a combination thereof.
4. A negative electrode, comprising: a negative electrode
collector; and a negative electrode active material coated on the
negative electrode collector, the negative electrode active
material comprising, a graphite core particle, a carbon layer
coating the graphite core particle, and metal particles dispersed
in the carbon layer; wherein the graphite core particle, the carbon
layer, and the metal particles form a metal-graphite composite
having a porosity of from 0.03 to 0.08 cc/g; wherein the graphite
core particle has a porosity of from 0 to 0.025 cc/g; and wherein
the carbon layer is coated to an average thickness of from 1 to 4
.mu.m.
5. A lithium secondary battery, comprising: a positive electrode
comprising a positive electrode active material; a negative
electrode comprising a negative electrode active material, the
negative electrode active material comprising, a graphite core
particle, a carbon layer coating the graphite core particle, and
metal particles dispersed in the carbon layer; a separator to
separate the positive electrode and the negative electrode; and an
electrolyte to immerse the positive and negative electrode; wherein
the graphite core particle, the carbon layer, and the metal
particles form a metal-graphite composite having a porosity of from
0.03 to 0.08 cc/g; wherein the graphite core particle has a
porosity of from 0 to 0.025 cc/g; and wherein the carbon layer is
coated to an average thickness of from 1 to 4 .mu.m.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Korean Patent Application
No. 10-2008-0120514, filed Dec. 1, 2008, the disclosure of which is
hereby incorporated herein, by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
Aspects of the present invention relate to a negative electrode
active material, a negative electrode having the same, and a
lithium secondary battery including the negative electrode.
2. Description of the Related Art
While lithium metals are commonly used as negative electrode active
materials, these metals can cause a short circuit in a battery, due
to the formation of dendrites, resulting in a risk of explosion.
For this reason, carbonaceous materials have recently been used in
place of lithium metals, as negative electrode active
materials.
Carbonaceous negative electrode active materials for a lithium
battery include crystalline carbons, such as natural graphite and
artificial graphite, and amorphous carbons, such as soft carbon and
hard carbon. Amorphous carbon has a high capacity, but is highly
irreversible in a charge/discharge cycle. Crystalline carbon, e.g.,
graphite, has a sufficiently high theoretical capacity (372 mAh/g)
to be used as a negative electrode active material, but is rapidly
degraded, resulting in a short lifespan. Also, a carbonaceous
active material cannot be use in a negative electrode of a high
capacity lithium battery, since its theoretical capacity does not
exceed 372 mAh/g.
In an effort to overcome these problems, metal-graphite composite
negative electrode active materials, including graphite and, for
example, aluminum (Al), germanium (Ge), silicon (Si), tin (Sn),
zinc (Zn) and/or lead (Pb), are being actively studied for use in
lithium batteries. However, in such composite negative electrode
active materials, lithium ions may be intercalated into inorganic
particles, such as Si or Sn, included in the composite negative
electrode active material, during charging, and thus, the inorganic
particles may expand by about 300 to 400%.
Further, when lithium ions are deintercalated during discharging,
the inorganic particles contract. As the charge/discharge cycles
are repeated, the conductivity of such active materials may be
decreased, due to the volume changes of the inorganic particles. In
addition, such a negative electrode active material may separate
from a negative electrode collector, resulting in a drastic
decrease in cycle-life.
SUMMARY OF THE INVENTION
Aspects of the present invention provide a negative electrode
active material having improved cycle-life characteristics, a
negative electrode having the same, and a lithium secondary battery
including the negative electrode. The negative electrode active
material can include a metal-carbon composite that is resistant to
volume changes, during charging and discharging.
According to one aspect of the present invention, provided is a
negative electrode active material including: a graphite core
particle; a carbon layer coating the graphite core particle; and
metal particles dispersed in the carbon layer.
According to another aspect of the present invention, provided is a
negative electrode including a negative electrode collector that is
coated with the negative electrode active material.
According to still another aspect of the present invention,
provided is a lithium secondary battery including a positive
electrode having a positive electrode active material, the negative
electrode, a separator separating the positive electrode from the
negative electrode, and an electrolyte.
According to aspects of the present invention, the negative
electrode active material may include a metal-carbon composite
having a porosity of from 0 to 0.08 cc/g.
According to aspects of the present invention, the negative
electrode active material may have a porosity of from 0 to 0.03
cc/g.
According to aspects of the present invention, the graphite core
particle may include a metal-carbon composite having a porosity of
from 0 to 0.07 cc/g.
According to aspects of the present invention, the graphite core
particle may have a porosity of from 0 to 0.025 cc/g.
According to aspects of the present invention, the carbon layer may
be coated to a thickness of 1 to 4 .mu.m.
Additional aspects and/or advantages of the invention will be set
forth in part in the description which follows and, in part, will
be obvious from the description, or may be learned by practice of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
These and/or other aspects and advantages of the present invention
will become apparent and more readily appreciated from the
following description of the exemplary embodiments, taken in
conjunction with the accompanying drawings, of which:
FIG. 1 is a schematic cross-sectional view of a negative electrode
active material, according to an exemplary embodiment of the
present invention;
FIG. 2 is a schematic cross-sectional view of a conventional
negative electrode active material;
FIG. 3A is a micrograph showing a cross-section of an exemplary
negative electrode active material, according to aspects of the
present invention; and
FIG. 3B is a micrograph showing a cross-section of a conventional
negative electrode active material.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
Reference will now be made in detail to the exemplary embodiments
of the present invention, examples of which are illustrated in the
accompanying drawings, wherein like reference numerals refer to the
like elements throughout. The exemplary embodiments are described
below, in order to explain the aspects of the present invention, by
referring to the figures.
FIG. 1 is a schematic cross-sectional view of a negative electrode
active material 100, according to an exemplary embodiment of the
present invention. Referring to FIG. 1, the negative electrode
active material 100 includes a graphite core particle 110, a carbon
layer 130 coating the graphite core particle 110, and metal
particles 120 dispersed in the carbon layer 130. The core particle
110, carbon layer 130, and metal particles 120 can be collectively
referred to as a metal-graphite composite.
The graphite core particle 110 is capable of reversibly
intercalating and deintercalating lithium ions. The graphite core
particle 110 may be formed of at least one selected from the group
consisting of artificial graphite, natural graphite, graphitized
carbon fiber, graphitized mesocarbon microbeads, and amorphous
carbon.
The graphite core particle 110 may have an average diameter of from
about 1 to 20 .mu.m. When the average diameter of the graphite core
particle 110 is less than about 1 .mu.m, the metal particles 120
disposed in the carbon layer 130 may not be disposed on or around
the surface of the graphite core particle 110. In other words, the
metal particles 120 may not be sufficiently adhered to the graphite
core particle 110 and/or dispersed in the carbon layer 130.
However, when the average diameter of the graphite core particle
110 is more than about 20 .mu.m, the carbon layer 130 may not form
a uniform coating. The graphite core particle 110 may be included
in a negative electrode of a battery. The graphite core particle
110 may have a porosity of 0.07 cc/g, or less, and in particular,
0.025 cc/g, or less.
As described above, it is preferable that the graphite core
particle 110 according to the present invention has no pores. When
the porosity is more than 0.07 cc/g, the carbon layer 130 can seep
into the pores of the graphite core particle 110, thereby reducing
the thickness of the carbon layer 130. Thus, it may be difficult to
secure the metal particles 120 on or around the surface of the
graphite core particle 110, resulting in poor cycle-life
characteristics.
The porosity of the metal-graphite composite is measured with
powder of an electrode plate including the negative electrode
active material 100, which is disassembled in a discharged state,
from a battery cell. The measured porosity may be 0.08 cc/g, or
less, and in particular, 0.03 cc/g, or less.
The carbon layer 130 may be formed by annealing a polymer material,
such as a vinyl-based resin, a cellulose-based resin, a
phenol-based resin, a pitch-based resin, or a tar-based resin. The
carbon layer 130 may be in an amorphous state (relatively
un-graphitized). When the carbon is in an amorphous state, an
electrolyte may not significantly penetrate the carbon layer 130,
resulting in an increase in the charge/discharge efficiency of the
negative electrode active material 100.
The carbon layer 130 has a low reactivity to the electrolyte and
acts as a reaction protection layer that prevents the dissolution
of the electrolyte, since it coats the metal particles 120, which
have a relatively high reactivity to the electrolyte. In addition,
the metal particles 120 are dispersed in the carbon layer 130, and
particularly, on or around the surface of the graphite core
particle 110, so that the metal particles 120 are not separated
from the graphite core particle 110 and can all contribute to the
charge/discharge reaction.
Here, the carbon layer 130 may be formed to a thickness of from
about 1 to 4 .mu.m. When the thickness of the carbon layer 130 is
less than about 1 .mu.m, it may be difficult to dispose the metal
particles 120 on or around the surface of the graphite core
particle 110, which could result in a reduction in the cycle-life
characteristics of the negative electrode active material 100. When
the thickness of the carbon layer 130 is more than about 4 .mu.m,
the irreversible capacity of the negative electrode active material
100 may increase.
The metal particles 120 are formed of a metallic material capable
of forming an alloy with lithium and which can reversibly
intercalate and deintercalate lithium ions. The metal particles 120
have a higher ability to intercalate lithium ions than the graphite
core particle 110, so as to increase the total charge/discharge
capacity of the negative electrode active material.
The metal particles 120 can include at least one selected from the
group consisting of Cr, Sn, Si, Al, Mn, Ni, Zn, Co, In, Cd, Bi, Pb,
and V. According to some embodiments, the metal particles 120 are
formed of Si, which has the highest theoretical capacity (4017
mAh/g) of the group.
The metal particles 120 have an average particle size of from 0.01
to 1.0 .mu.m, for example, from 0.05 to 0.5 .mu.m. When the metal
particles 120 are smaller than 0.01 .mu.m, the agglomeration of the
metal particles 120 may be increased, resulting in non-uniform
dispersion of the metal particles 120 in the carbon layer 130. Such
non-uniformity may result in metal particles that are difficult to
use powder form, and may increase the dissolution of the
electrolyte, due to the large specific area thereof. On the other
hand, when the metal particles 120 are larger than 1.0 .mu.m, the
absolute volume of the metal particles 120 may increase during
charging/discharging, resulting in a reduction in capacity
retention characteristics.
The metal particles 120 are capable of forming an alloy with
lithium, and may reversibly intercalate lithium ions. As a result,
the capacity of the negative electrode active material 100 may be
increased, due to an increased intercalation/deintercalation of
lithium ions, as compared to a conventional carbonaceous negative
electrode active material.
The content of the metal particles 120 may be from 3 to 20 wt %,
with respect to 100 wt % of the negative electrode active material
100. When the content of the metal particles 120 is less than 3 wt
%, the energy density may be decreased. When the content of the
metal particle is more then 20 wt %, the charge/discharge
efficiency may be decreased.
FIG. 2 is a schematic cross-sectional view of a conventional
negative electrode active material 10. Referring to FIG. 2, the
conventional negative electrode active material 10 is formed of a
graphite core particle 11, metal particles 12 disposed on the
surface of the graphite core particle 11, and a carbon layer 13a
coating the graphite core particle 11 and the metal particles
12.
In the negative electrode active material 100, according to aspects
of the present invention, the metal particles 120 are dispersed in
the carbon layer 130, and thus, are disposed on or adjacent to the
surface of the graphite core particle 110. However, in the
conventional negative electrode active material 10, the metal
particles 12 are attached to the surface of the graphite core
particle 11, and then the graphite core particle 11 and the metal
particles 12 are coated with the carbon layer 13a.
Since the conventional graphite core particle 11 has a high
porosity, as shown in FIG. 2, the carbon layer 13a infiltrates
pores 14 of the graphite core particle 11. Thus, when the
conventional carbon layer 13a is formed of the same amount of
material as the carbon layer 130, the conventional carbon layer 13a
forms a thinner coating. As a result, the conventional carbon layer
13a fails to adequately secure the metal particles 12 to the
graphite core particle 11, resulting in reduced cycle life
characteristics.
FIG. 3A is a micrograph showing a cross-section of the negative
electrode active material 100, according to aspects of the present
invention, and FIG. 3B is a micrograph showing a cross-section of
the conventional negative electrode active material 10. Referring
to FIG. 3A, the negative electrode active material 100 includes the
graphite core particle 110, the carbon layer 130 coating the
graphite core particle 110, and the metal particles 120 dispersed
in the carbon layer 130. Since the graphite core particle 110 is
substantially nonporous, the thickness (a) of the carbon layer is
relatively large.
However, referring to FIG. 3B, the conventional negative electrode
active material 10 is formed of the graphite core particle 11, the
metal particles 12 disposed on the surface of the graphite core
particle 11, and the carbon layer 13a coating the graphite core
particle 11 and the metal particles 12. Since the carbon layer 13a
seeps into the pores 14, the carbon layer 13a has a much smaller
thickness (b). Due to the reduction of the thickness of the carbon
layer 13a, the metal particles 12 are not securely attached to the
graphite core particle 11, resulting in reduced cycle life
characteristics.
A method of forming a negative electrode active material, according
to aspects of the present invention, will now be described. First,
the graphite core particles 110 are prepared. The graphite core
particles 110 can be formed of any suitable material, such as
artificial graphite having a porosity of 0.07 cc/g, or less, and in
particular, 0.025 cc/g, or less.
A wet method is used to mix equal volumes of metal particles and a
carbon precursor, to form a precursor solution. The precursor
solution is coated on graphite core particles and then dried at
80.degree. C. The resultant material is annealed at 800.degree. C.
for 4 hours, in a nitrogen atmosphere, to form a metal-graphite
composite. The carbon precursor may include amorphous carbon and a
resin.
The metal particles can be selected from the group consisting of
Cr, Sn, Si, Al, Mn, Ni, Zn, Co, In, Cd, Bi, Pb, and V. In
particular, the metal particles can be Si particles having to an
average particle size of 200 nm, or less. The Si particles may be
formed by pulverization, using a ball mill, a jet mill, or an
attrition mill (attritor), but the present invention is not so
limited. Alternatively, the negative electrode active material may
be deflocculated prior to use.
The conventional method of forming a negative electrode active
material is similar to the method described above, except that in
the conventional negative electrode active material, porous
graphite core particles are used, and a certain amount of the
carbon layer is disposed in the pores, thereby decreasing the
thickness of the carbon layer. For this reason, the metal particles
are not strongly bound to the graphite core particles, resulting in
reduced cycle-life characteristics.
That is, in the conventional negative electrode active material,
the metal particles may affect other elements of a battery, due to
their expansion in the negative electrode, or may react with an
electrolyte. In addition, when the volume of the metal is reduced
during discharging, it may not be completely restored to its
original state. Accordingly, there is much space around the metal
particles, resulting in the electrical isolation of the active
materials, a reduction in capacity, and a degradation of battery
performance.
In contrast, the graphite core particles utilized in exemplary
embodiments of the present invention have a low porosity, allowing
the carbon layer to sufficiently coat the metal particles.
Therefore, it is easy to disperse the metal particles in the carbon
layer. The negative electrode active material, according to aspects
of the present invention, can strongly bind the metal particles,
due to the above-mentioned carbon layer, and thus, the volume
expansion thereof is reduced, thereby improving cycle-life
characteristics.
A lithium secondary battery, according to aspects of the present
invention, will now be described. The battery includes an electrode
assembly disposed in a can with an electrolyte. The electrode
assembly includes a negative electrode, a positive electrode, and a
separator disposed there between.
The negative electrode includes a negative electrode collector,
which is coated with a negative electrode active material. The
negative electrode collector may be formed of copper or a copper
alloy. The negative electrode collector may be in the form of a
foil, a film, a sheet, a punched-type, a porous-type, or a
foamy-type.
Then, the lithium secondary battery having the negative electrode
active material according to the present invention includes a
positive electrode having a positive electrode active material, a
negative electrode having a negative electrode active material, a
separator separating the positive electrode from the negative
electrode, and an electrolyte. Here, the negative electrode active
material is the same as described above.
The positive electrode may include a positive electrode collector
that is coated with a positive electrode active material that can
reversibly intercalate lithium ions. Examples of the positive
electrode active material include lithium-transition metal oxides,
such as LiCoO.sub.2, LiNiO.sub.2, LiMnO.sub.2, LiMn.sub.2O.sub.4,
and LiNi.sub.1-x-yCO.sub.xM.sub.yO.sub.2 (0.ltoreq.x.ltoreq.1,
0.ltoreq.y.ltoreq.1, 0=x+y.ltoreq.1, and M is metal such as Al, Sr,
Mg or La). However, the present invention is not so limited.
The positive electrode collector may be formed of aluminum or an
aluminum alloy. The positive electrode collector may be in the form
of a foil, a film, a sheet, a punched-type, a porous-type, or a
foamy-type. The separator may be formed of a resin layer, such as a
polyethylene or a polypropylene; or a porous layer formed by
coupling a ceramic material and a binder, but the present invention
is not so limited.
The electrolyte includes a non-aqueous organic solvent, e.g., a
carbonate, an ester, an ether, or a ketone. The carbonate may
include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl
carbonate (DPC), methyl propyl carbonate (MPC), ethylpropyl
carbonate (EPC), methylethyl carbonate (MEC), ethylene carbonate
(EC), propylene carbonate (PC), or butylene carbonate (BC). The
ester may include butyrolactone (BL), decanolide, valerolactone,
mevalonolactone, caprolactone, n-methyl acetate, n-ethyl acetate,
or n-propyl acetate. The ether may include dibutyl ether, and the
ketone may include polymethylvinyl ketone. However, the present
invention is not so limited.
When the non-aqueous organic solvent is a carbonate-based organic
solvent, it may be formed by mixing a cyclic carbonate with a chain
carbonate, at a volume ratio of from 1:1 to 1:9, or more
specifically, from 1:1.5 to 1:4. In these ranges, the electrolyte
may exhibit proper performance.
The electrolyte may further include an aromatic hydrocarbon-based
organic solvent, as well as the carbonate-based solvent. The
aromatic hydrocarbon-based organic solvent may be an aromatic
hydrocarbon-based compound. Specifically, the aromatic
hydrocarbon-based organic solvent may include benzene,
fluorobenzene, chlorobenzene, nitrobenzene, toluene, fluorotoluene,
trifluorotoluene, or xylene. The volume ratio of the
carbonate-based solvent to the aromatic hydrocarbon-based solvent
may be from 1:1 to 30:1. In these ranges, the electrolyte mixture
may exhibit proper performance.
In addition, the electrolyte includes a lithium salt, which acts as
a source of lithium ions for the basic operation of the battery.
The lithium salt includes at least one selected from the group
consisting of LiPF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAsF.sub.6,
LiClO.sub.4, LiCF.sub.3SO.sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiAlO.sub.4, LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1F.sub.2x+1SO.sub.2)(C.sub.yF.sub.2x+1SO.sub.2)
(x and y are natural numbers), and LiSO.sub.3CF.sub.3.
The lithium salt may have a concentration of from 0.6 to 2.0M, and
more specifically, from 0.7 to 1.6M. When the concentration of the
lithium salt is less than 0.6M, the conductivity of the electrolyte
may decrease. When the concentration of the lithium salt is more
than 2.0M, the viscosity of the electrolyte may be increased,
decreasing the mobility of the lithium ions.
The positive electrode, the negative electrode, and the separator
are stacked together and then wound into a jellyroll-type shape, to
form the electrode assembly. The electrode assembly is inserted
into the can, and then the electrolyte is injected into the can,
completing the lithium secondary battery. The can may be, for
example, cylindrical, rectangular, or pouch-type.
Hereinafter, examples and comparative examples of the present
invention will be described. The examples are provided only to
understand the aspects of the present invention, and thus, the
present invention is not limited to the following examples.
Example 1
LiCoO.sub.2, a polyvinylidene fluoride (PVDF) binder, and a
conductive carbon material were mixed, at a weight ratio of 92:4:4,
to form a mixture. The mixture was dispersed in
N-methyl-2-pyrollidon to form a positive electrode slurry. The
slurry was coated on 20 .mu.m thick aluminum foil, dried, and
rolled to form a positive electrode.
Silicon particles were mixed with pitch-based carbon to form a
mixture. The mixture was then coated on graphite core particles,
thereby forming a metal-graphite composite including graphite core
particles coated with a carbon layer having the silicon particles
dispersed therein.
The metal-graphite composite was dry-mixed with 20 .mu.m artificial
graphite, at a ratio of 7:3. The resultant was mixed with, a
styrene-butadiene rubber binder and a carboxymethylcellulose
thickener, at a weight ratio of 96:2:2, and then dispersed in
water, thereby forming a slurry. The slurry was coated on a 15
.mu.m copper foil, dried, and rolled to form a negative electrode
having a negative electrode active material coating.
The core graphite particles had a porosity of 0.025.+-.0.01 cc/g,
or less. The carbon layer was coated on the core graphite particles
to a thickness of about 3 .mu.m. The porosity of the metal-graphite
composite was 0.03.+-.0.02 cc/g. A 20 .mu.m separator formed of a
polyethylene (PE) film was introduced between the positive and
negative electrodes, and the resulting structure was wound,
pressed, and inserted into a cylindrical can. Then, an electrolyte
was injected into the cylindrical can, thereby manufacturing a
lithium secondary battery.
Example 2
The method of Example 1 was repeated, except that core graphite
particles having a porosity of 0.07.+-.0.03 cc/g were coated with
a1 .mu.m thick carbon layer, to form a metal-graphite composite.
The metal-graphite composite had porosity of 0.08.+-.0.02 cc/g.
Then, a lithium secondary battery was manufactured using the
negative electrode material, in the same manner as in Example
1.
Comparative Example 1
LiCoO.sub.2, a PVDF binder, and a conductive carbon material were
mixed at a ratio of 92:4:4. The resulting mixture was dispersed in
N-methyl-2-pyrollidon, to form a slurry. The slurry was coated on
20 .mu.m thick aluminum foil, dried, and rolled to form a positive
electrode. Silicon particles were attached to the surfaces of
graphite core particles, and then the graphite core particles were
coated, with the same amount of pitch-based carbon as in Example 1,
thereby forming a comparative metal-graphite composite, which was
used to form a negative electrode active material.
That is, in order to form the carbon layer, in Example 1, the
silicon particles were mixed with the pitch-based carbon, and then
the graphite core particles were coated with the pitch-based
carbon/silicon particle mixture, forming a carbon layer on the
graphite core particles. However, in Comparative Example 1, the
silicon particles were attached to the surfaces of the graphite
core particles and then coated with the pitch-based carbon, to form
a comparative metal-graphite composite.
The comparative metal-graphite composite was dry-mixed with 20
.mu.m artificial graphite, at a ratio of 7:3. The resultant mixture
was mixed with a styrene-butadiene rubber binder, and a
carboxymethylcellulose thickener, at a weight ratio of 96:2:2, and
then dispersed in water, thereby forming a slurry. The slurry was
coated on 15 .mu.m thick copper foil, dried, and rolled to form a
comparative negative electrode having a negative electrode active
material coating.
In Comparative Example 1, the core graphite had a porosity of
0.14.+-.0.02 cc/g, and the carbon layer had a thickness of about
200 .mu.m. The metal-graphite composite had a porosity of
0.15.+-.0.03 cc/g. A 20 .mu.m thick separator formed of a
polyethylene (PE) film was introduced between the positive and
negative electrodes, and the resultant structure was wound,
pressed, and inserted into a cylindrical can. An electrolyte was
injected into the cylindrical can, thereby manufacturing a
comparative lithium secondary battery.
Comparative Example 2
The method of Comparative Example 2 was repeated, except that core
graphite particles having a porosity of 0.09.+-.0.03 cc/g were
used, and a 300 .mu.m thick carbon layer was formed. The resulting
comparative metal-graphite composite had a porosity of 0.10.+-.0.02
cc/g.
The lithium batteries in Examples 1 and 2 and Comparative Examples
1 and 2 were charged to 4.35V and discharged to 2.5V, for 100
cycles, at a current density of 1 C. Then, the charge/discharge
efficiencies at the 1.sup.st cycle (ratio of discharge capacity to
a charge capacity) and the capacity retention ratio at the
100.sup.th cycle to the 1.sup.st cycle, were measured. The
measurement results are shown in Table 1, below.
TABLE-US-00001 TABLE 1 Porosity of Charge/ Capacity Metal-Graphite
discharge Retention Composite Efficiency at the Ratio at the (cc/g)
1.sup.st cycle (%) 100.sup.th cycle (%) Example 1 0.03 90 80
Example 2 0.08 89 78 C. Example 1 0.15 87 45 C. Example 2 0.10 88
60
From the results shown in Table 1, it can be noted that Examples 1
and 2 exhibited better charge/discharge efficiencies at the
1.sup.st cycle than Comparative Examples 1 and 2. Further, it can
be noted that Examples 1 and 2 exhibited higher capacity retention
ratios at the 100.sup.th cycle than Comparative Examples 1 and
2.
In more detail, when the negative electrode active materials of
Comparative Examples 1 and 2 were repeatedly charged and
discharged, the metal particles likely expanded, and/or became
dislodged from the corresponding negative electrodes. In addition,
when the metal particle volume was decreased during discharging, it
likely was not completely restored to its original state. Thus,
gaps were likely formed around the meal particles, insulating the
metal particles, and resulting in the decreases in electrical
capacity and battery performance. However, in the negative
electrode active material according Examples 1 and 2, the metal
particles were strongly bound by the carbon layer, resulting in
less expansion and improved cycle-life characteristics.
Consequently, a negative electrode active material, according to
aspects of the present invention, has a reduced volume change, and
thus, results in a secondary battery having improved cycle-life
characteristics.
Although a few exemplary embodiments of the present invention have
been shown and described, it would be appreciated by those skilled
in the art that changes may be made in these exemplary embodiments,
without departing from the principles and spirit of the invention,
the scope of which is defined in the claims and their
equivalents.
* * * * *